The bismaleimides (BMI) are a type of thermosetting compounds that have been used in a variety of applications. BMIs have been particularly useful in adhesive applications. Bismaleimides are curable, meaning that they are capable of polymerization to yield cross-linked resins. BMIs can be homocured in the presence of free radicals or photoinitiators, or they can be combined with other free-radical curing monomers (e.g., acrylates, methacrylates, styrenics, vinyl ethers, vinyl esters, allyl monomers, olefins, and the like). They may also be cured in the presence of co-monomers, e.g. via Diels-Alder or Michael addition mechanisms.
Commercially available BMI thermoset compounds and BMI-containing compositions are noted for their high modulus, and excellent resistance to thermal degradation. However, these thermosets are also well known for brittleness. The utility of the bismaleimide class of thermosets could be vastly improved if less brittle formulations could be achieved that retain the desirable thermal and elastic properties.
A few methods have been described in the art to decrease the brittleness of bismaleimide thermosets. The well-known Michael addition reaction, using aromatic or aliphatic diamines is one of these methods. Michael addition has been used to reduce the cross-link density and therefore to improve the toughness of these thermosets. The Michael addition approach to toughen BMI resins, however, has at least one significant limitation. Michael addition is thermally reversible and the amine extended bismaleimides will revert to the starting materials in the vicinity of 300° C. This thermal reversion or retro-Michael addition is unacceptable for any adhesive end-use that requires thermal resistance.